Your TV needs to have a 16:9 setting. I don't pretend to know anything about graphical fidelity. All I know is I set up my Wii to display in 16:9 in "Wii Settings" and then whenever I turn on the TV for the Wii I have to press the 16:9 button on my remote.
To be honest, if I forget to hit the button on the remote and play in 4:3, I really don't notice the difference. I'll have to look more closely.